¹²⁹Xe NMR Chemical Shifts in Zeolites:  Effect of Loading Studied by Monte Carlo Simulations

Abstract

The NMR chemical shift of 129Xe adsorbed in the cavities and channels of zeolites depends on the crystal type (structure and chemistry) and on the loading of Xe. The dependence of the 129Xe shift on Xe loading has frequently been interpreted qualitatively in terms of increasing Xe−Xe interactions, but this dependence can be quantitatively and predictively modeled using computer simulations. We show that Monte Carlo simulations of Xe inside Omega, mordenite, AlPO4-5, L, Y, ZK-4, silicalite, and VPI-5 give Xe−Xe pair correlation functions that are entirely consistent with the shift behavior reported in the literature. The sensitivity of the shift dependence on loading correlates well with the calculated accessible volume; this correlation distinguishes zeolites even with similar unidirectional channels and indicates that an estimate of the available porosity of a zeolite can be made from such data. There is reasonable (but not in all cases quantitative) agreement between the predicted and experimental zeolite shift contribution.

Agreement between Analytical Theory and Molecular Dynamics Simulation for Adsorption and Diffusion in Crystalline Nanoporous Materials

“…A significant body of literature has been devoted to investigating the phenomenon of adsorption and diffusion in nanoporous materials. − …”

129Xe NMR overview of xenon physisorbed in porous solids

“…14 Monte Carlo simulation or molecular dynamics simulation of a Xe atom in a 6-12 potential have been performed on different zeolites: Y, silicalite and mordenite. 15,16 The xenon mobility is a very important parameter which must be taken into account when interpreting the data (see the section 'Diffusion of xenon and of other adsorbates', below). At room temperature, the rapid movement of xenon atoms through large-pore zeolites usually limits resolvable structural details to macroscopic dimensions.…”